In electric double layer capacitors using a polarizable electrode for a positive electrode and a negative electrode respectively, charge and discharge under high load is possible due to absorption and desorption of cations and anions at the electrode surfaces during the charge and discharge processes. This is because the polarizable electrodes are made by using powder or fabric of activated carbon having a larger specific surface area. The polarizable electrodes are made by kneading the activated carbon with carbon black as a conductive agent, and a binder, as required, and then molding the kneaded material.
Additionally, by using a non-aqueous electrolyte including a non-aqueous solvent as a solvent for dissolving a supporting salt, a charging voltage of the electric double layer capacitor can be set higher, thereby increasing the energy density of the capacitor.
As for a typical non-aqueous solvent, a cyclic carbonate such as ethylene carbonate (EC), propylene carbonate (PC), or butylene carbonate (BC), and a cyclic ester such as γ-butyrolactone (γ-BL) can be mentioned, for example. The non-aqueous electrolyte is prepared by dissolving a quaternary ammonium salt such as N,N,N,N-tetraethylammonium-tetrafluoroborate (TEA-BF4) or N,N,N-triethyl-N-methylammonium-tetrafluoroborate (TEMA-BF4) in these non-aqueous solvents.
However, due to a high flammability of the non-aqueous electrolyte containing a non-aqueous solvent as a main component, safety for the electric double layer capacitors including such non-aqueous electrolyte is concerned. Additionally, declining reliability of the electric double layer capacitors is becoming a problem, since the non-aqueous solvent is apt to cause a side reaction with an activated carbon or carbon black.
In order to decrease flammability of the non-aqueous electrolyte, there has been proposed to use a phosphazene compound as a non-aqueous solvent, and to admix a cyclic carbonate or a cyclic ester as necessary. For example, Japanese Laid-Open Patent Publication No. 2001-217155 proposes an electrolyte obtained by mixing a quaternary ammonium salt, an organic solvent, and a phosphazene compound having a potential window of 0.5 to 4.5V relative to a lithium reference electrode. Also, in Japanese Laid-Open Patent Publication No. 2001-217156, an electrolyte including 0.5 mol/liter of quaternary ammonium salt, and a phosphazene compound having a conductivity of 2.0 mS/cm or more has been proposed, for example. Further, in Japanese Laid-Open Patent Publication No. 2001-217157, an electrolyte including a quaternary ammonium salt, and a phosphazene compound having a dielectric constant of 15 or more (25° C.) and a viscosity of 20 cP or below has been proposed, for example.
The ammonium salts used in the electrolytes described in each of the above publications are electrolyte salts solid at room temperature typically represented by N,N,N,N-tetraethylammonium-tetrafluoroborate (TEA-BF4).